1. Field of the Invention
The present invention relates to a nitride-based semiconductor light-emitting device using a nitride-based semiconductor represented by the general formula InxAlyGa1-x-yN (x+y+z=1, 0≦x≦1, 0≦y≦1 and 0≦z≦1) and a manufacturing method thereof.
2. Description of the Background Art
The conventional nitride-based semiconductor light-emitting devices are mostly fabricated by layering a nitride-based semiconductor layer on a sapphire substrate. In recent years, however, for the reduction of manufacturing cost of the light-emitting device, the use of silicon (Si) substrate have become common as the silicon substrate is less expensive and usable by a large area.
FIG. 12 shows a schematic perspective of a conventional nitride-based semiconductor light-emitting device using an Si substrate. The nitride-based semiconductor light-emitting device includes on an Si substrate 100, an AlN buffer layer 101, an n-type GaN layer 102, an InGaN light-emitting layer 103, a p-type AlGaN carrier block layer 104 and a p-type GaN contact layer 105 successively layered, a translucent electrode 106 formed on p-type GaN contact layer 105, and an n-type-use electrode 107 formed on n-type GaN layer 102. Further, a p-type-use pad electrode 108 is formed on translucent electrode 106 and an n-type-use pad electrode 109 is formed on n-type-use electrode 107.
In this nitride-based semiconductor light-emitting device, however, a part of light emitted from InGaN light-emitting layer 103 is directed towards Si substrate 100 and absorbed by Si substrate 100, whereby light extraction efficiency of the light emitted from InGaN light-emitting layer 103 is decreased.
It may be possible to form a reflective film on Si substrate 100 from materials such as metals to prevent the incidence of light to Si substrate 100 and to extract the light from the side surface of the semiconductor light-emitting device in the same manner as in a device with a sapphire substrate. However, the nitride-based semiconductor layer cannot be formed thick as the difference in thermal expansion coefficient between nitride-based semiconductor layers causes crack. Hence, it is impossible to improve light extraction efficiency by extracting light from the side surface of the nitride-based semiconductor layer using this portion to let the light emitted from the light-emitting layer pass through for the extraction.
To solve the problem as described above, Japanese Patent Laying-Open No. 2000-196152 discloses a light-emitting device including a p-type GaN semiconductor layer with irregularities and a light-emitting device including a light extraction layer with an irregular surface formed on a p-type GaN semiconductor layer via a transparent electrode. When the film thickness of the p-type GaN semiconductor layer is increased for the formation of irregularities, many cracks are formed in the p-type GaN semiconductor layer thereby increasing the driving voltage of the device. It is assumed that when the p-type GaN semiconductor layer is grown at a high temperature and brought back to room temperature after the growth, tensile stress applied on the p-type GaN semiconductor layer causes cracks and that the thickness of the film of the p-type GaN semiconductor layer also contributes to the liability of crack formation. In addition, the p-type GaN semiconductor layer has a further disadvantage as it is unlikely to be low in resistance essentially and because of the film thickness, which leads to a further increase in the driving voltage of the device. Further, even when the transparent electrode is formed on the p-type GaN semiconductor layer, ohmic characteristics between the p-type GaN semiconductor layer and the transparent electrode is not favorable and contact resistance tends to become high, whereby the driving voltage of the device increases.